Light emitting diodes (hereinafter sometimes referred to as LED) are semiconductor diodes that emit light and convert electric energy to ultraviolet light or visible light. In particular, in order to utilize visible light, LED light emitting devices comprising a light emitting chip formed of a luminescent material such as GaP, GaAsP, GaAlAs, GaN, or InGaAlP which has been sealed with a transparent resin have been widely used.
Display-type LED light emitting devices comprising a luminescent material that is fixed on an upper surface of a printed board or a metallic lead and is sealed with a resin case on which numerals or letters are provided have also been extensively used. A method may also be adopted in which various phosphor powders are incorporated in the front surface of the light emitting chip or the front part of the resin for use in sealing of the light emitting chip to properly adjust the color of light emitted. That is, regarding the color of light emitted in the LED light emitting device, luminescence in a visible light range from blue to red can be reproduced depending upon the contemplated application of the LED light emitting device.
Light emitting diodes are semiconductor elements and have a prolonged service life and a high reliability and, thus, when used as a light source, can reduce the necessary frequency of exchange thereof. Accordingly, the light emitting diodes have been extensively used as components for constituting various display devices such as portable communication devices, peripheral devices of personal computers, OA equipment, electrical household machinery and equipment, audio equipment, various switches, and light source display plates for backlights.
Recently, chromatic sensation of users of the various display devices described above has been improved, and the function of reproducing subtle shades of color with higher definition and uniform appearance of LED light emitting devices have also become required of various display devices. In particular, in LED light emitting devices that emit white light, use in backlights of cellular phones, in-car lamps and the like has recently been significantly expanded, and, thus, significant growth of use of the LED light emitting devices that emit white light, as an alternative to fluorescent lamps, in the future is expected. In LED light emitting devices that emit white light, to bring an end to the expectation, various improvements have been attempted to realize high color rendering properties of the white light and a uniform appearance. Further, in the RHoS regulation which has come into force in European Union (EU) on July, 2006, the use of mercury is prohibited, and it is considered that the conventional fluorescent lamps are replaced by white light emitting LED lamps not using mercury.
LED devices, which emit white light and are currently in widespread use or experimental use, are classified into a type in which a blue light emitting diode is combined with a yellow light emitting phosphor and, in some cases, further a red phosphor (hereinafter referred to as type 1) and a type in which an ultraviolet light or purple light emitting diode is combined with blue, yellow, and red phosphors (hereinafter referred to as type 2).
At the present time, type 1 has higher brightness than type 2 and is in the most widespread use. Materials such as cerium activated yttrium aluminate phosphors (hereinafter referred to as YAGs), cerium activated terbium aluminate phosphors (hereinafter referred to as TAGs), and alkaline earth silicate phosphors (hereinafter referred to as BOSS) have been put into practical use as yellow phosphors for type 1.
Among them, YAG and BOSS are phosphors that are well known before use in combination with light emitting diodes and, up to now, have been used or attempted to be applied to applications such as flying spot scanners and fluorescent lamps. These phosphors have already been put into practical use, for example, in backlights for cellular phones, but are daily improved with a view to aiming at further expansion, for example, to illumination lamps and head lamps in automobiles. Regarding the BOSS phosphors, for example, patent document 1 discloses an improvement in these phosphors. For aluminate phosphors such as YAG and TAG, for example, patent documents 2 to 21 disclose an improvement in these phosphors.